1. Field of the Invention
This invention concerns ventilators for inducing or assisting lung function in human patients. The invention is especially concerned with ventilators for use in resuscitation, rescue breathing and patient transport.
2. Background Discussion and the Prior Art
Ventilators for such purposes have been available for many years and two main types are in widespread use today: automatic ventilators in which the lung ventilation parameters (tidal volume, phase durations) are determined by control settings of the ventilator; and manual ventilators in which the ventilation parameters are directly controlled by the user.
Automatic ventilators have the advantage that when once the control settings for required parameters have been established--in some cases once and for all during manufacture--efficient lung ventilation can be consistently maintained through repeated inhalation/exhalation cycles without intervention of the user. In manually controlled ventilators, the delivered tidal volume of breathable gas and the inhalation and exhalation times for each cycle are determined individually by action of the user, requiring continual attention and the exercise of judgement by the user.
Automatic ventilators offer unquestionable advantages for rescue breathing and patient transport but can have some disadvantages in one-man resuscitation in cases of cardiac arrest, wherein the common resuscitation technique (cardiac pulmonary resuscitation, CPR) involves applying a sequence of chest compressions interrupted at regular intervals by one or more lung ventilations: typically two lung ventilation cycles followed by 15 chest compressions followed by two further lung ventilation cycles, and so on. With an automatic ventilator continuously delivering a train of pulses of breathable gas to a face mask at a constant rate, this resuscitation technique demands that the face mask be applied to the patient for two ventilation cycles and then removed; the required number of chest compressions be then applied; and thereafter the face mask be reapplied to the patient: all in synchronism with the operation of the ventilator so that this delivers a breathable gas pulse immediately the mask has been applied or re-applied to the patient. Especially in the case of one-man resuscitation, this requires dexterity and skill on the part of the rescuer to achieve the required synchronisation of the manual operations with the cyclic operation of the ventilator and therefore many rescuers opt to use a manual ventilator in such circumstances; the manual ventilator allows the rescuer to initiate delivery of a breathable gas pulse when required to fit in with the sequence of chest compressions and mask application.
A need can therefore be perceived for a rescue/resuscitation ventilator that can be more conveniently used in CPR than existing ventilators, by having the ability to deliver instantly on demand a single controlled ventilation cycle of predetermined tidal volume and duration, so avoiding the synchronisation requirements of the automatic ventilator and also the disadvantages inherent in the use of existing manual ventilators.
There have already been proposals for ventilators that are switchable between an automatic mode of operation and a fully manually controlled mode. However, in the manually controlled mode, these ventilators exhibit the above discussed disadvantages of manually controlled ventilators and, thus, cannot offer a satisfactory solution to the problem.